This invention concerns improvements in or relating to bending of thermoplastic tubes. More specifically, the invention provides an improved method or apparatus for bending of thermoplastic tubes. The invention has particular application t bending of thin wall large bore thermoplastic tubes. As used herein, the term xe2x80x9ctubexe2x80x9d includes hoses, pipes or similar conduits, whether of single or multi-layer construction.
Thermoplastic tubes are used for many applications, for example in the automotive field where the tube may have to be of a specific configuration with one or more bends to provide the necessary fluid flow connection.
It is desirable to form such tubes having smooth bends with minimum distortion of the tube wall giving non-turbulent flow to maintain the optimum flow characteristics through the tube and produce an aesthetically pleasing appearance.
A method and apparatus for shaping thermoplastic tube is described in UK Patent No. 2249750-B in which the tube is pre-formed to a required bend configuration prior to effecting contactless heating of the bend region by pulsed emission of infra-red radiation to soften the thermoplastic material followed by air cooling to set the thermoplastic material and retain the required bend configuration.
The tube is pre-shaped on a jig with appropriately positioned sources of infra-red radiation and air to carry out the heating and cooling stages with the tube held stationary on the jig.
With this method and apparatus, it is possible to produce tubes of complex shape with multiple bends in different planes being formed simultaneously. This method is particularly suitable for shaping small bore thermoplastic tubes without requiring any internal support such as a separate removable mandrel to prevent the tube wall collapsing.
Problems can arise however in trying to shape thin wall large bore thermoplastic tube by such method and apparatus. In particular, there is a much greater tendency for thin wall large bore tube to flatten across their section at 90xc2x0 to the bend axis resulting in an elliptical shaped bore or, in extreme cases, the tube may fold or even collapse on it itself producing an unacceptably deformed tube.
As a result, it is still common for thin wall large bore tubes to be provided with an internal solid mandrel which is flexible to allow the tube to be formed to the required shape and which is removed after the tube is set in the required shape. The use of such mandrels adds to production costs and restricts the shapes that can be produced without completely stopping the tube creasing. Thus, sufficient clearance has to be allowed for insertion and removal of the mandrel. As a result, some collapse of the tube wall may occur during forming and the formed lube may be deformed by removal of the mandrel. In addition, lubricants which are possible undesirable contaminants may have to be used to assist removal of the mandrel.
It is an object of the present invention to provide an improved method and apparatus for shaping thermoplastic tubes which enables bends to be formed in a wider range of tube sizes than previously.
It is a particular desired object of the present invention to provide an improved method and apparatus for shaping thin wall large bore thermoplastic tubes.
It is a further preferred object of the present invention to provide an improved method and apparatus for shaping thin wall large bore thermoplastic tubes which enables bends to be formed without significantly changing the bore section.
It is yet another preferred object of the present invention to provide an improved method and apparatus for shaping thin wall large bore thermoplastic tubes which enables bends to be formed having a small radius or angle.
It is still further preferred object of the present invention to provide an improved method and apparatus for shaping thin wall large bore thermoplastic tubes to provide multiple bends in different planes enabling complex shapes to be produced.
According to one aspect of the present invention there is provided a method of forming a bend having a required configuration in a tube of thermoplastic material comprising locally heating a region of the tube in which a bend is to be formed until the plastic material becomes sufficiently plastic for subsequent shaping to a required bend configuration, positioning the tube between relatively movable formers providing external support around a section of the heated region of the tube, relatively moving the formers to form progressively the required bend configuration while simultaneously providing internal support for the tube by a pressure differential between the inside and outside of the tube, and cooling the tube to set thermoplastic material and retain the required bend configuration.
By this invented method, the formation of the bend with the required configuration is achieved without deforming or collapsing so as to maintain substantially the original cross-section of the tube. This is made possible in the invented method by supporting the tube internally and externally during the bending operation.
In particular, the relatively movable formers enclose a section of the tube at the point where the bending force is applied and this enclosed section moves progressively along the heated region as the bend is developed by the relative movement of the farmers.
Preferably, the pressure differential is created by the provision of a fluid medium, conveniently compressed air, within the tube during the forming operation. The fluid medium may be confined within the tube by appropriate sealing means at each end of the tube. Alternatively, the fluid medium may be confined in an inflatable bladder inserted into the tube.
Advantageously, cooling is effected externally and/or internally, for example by a fluid medium, such as a gas or liquid, caused to flow over the exterior andor through the interior of the tube by any suitable means,such as fans, blowers, sprays etc.
Preferably, the pressure differential is maintained during the cooling stage to prevent the tube deforming or collapsing before the thermoplastic material has set. This may be achieved by the use of compressed air both in the forming and cooling stages.
Advantageously, external support on the inside of the bend is maintained as the bend is developed. For example, the tube may be progressively formed around a former having a profile corresponding substantially to the profile of the required bend.
Preferably, external support on the outside of the bend is provided at the point where bending forces are applied and moves along the tube as the bend is developed. For example, a former for the outside of the bend may move around the former for the inside of the bend. In this way, as the bend is developed, the section of the tube being formed is totally enclosed.
The formers may be relatively movable to form the bend in one or more planes. More than one bend may be formed at spaced apart positions along the length of the tube to produce complex configurations. Multiple bends may be in the same or different planes.
It may be possible with the invented method to form two bends simultaneously with further bends produced in pairs sequentially along the length of the tube as desired.
According to another aspect of the present invention there is provided apparatus for forming a bend in a tube of thermoplastic material comprising means for heating a pre-determined region of a tube to soften the plastic material for subsequent shaping, means for shaping the heated region of the tube to form a bend having a required configuration, and means for cooling the tube to set the thermoplastic material and retain the required bend configuration, the shaping means including relatively movable former means for externally supporting a section of the heated region of the tube at the point of bending and means for internally supporting the tube during the being operation by a pressure differential between the inside and outside of the tube.
The former means provide localised external support around the tube as the bend is formed, with the pressure differential providing internal support to prevent the tube collapsing or deforming.
Preferably, the former means comprises a fixed former having a profile corresponding to the inside of the bend and a former movable around the fixed former to conform the tube to the profile of the fixed former and provide support for the outside of the bend at the point of bending.
In this way, the section of the tube where the bending forces are applied is totally enclosed by the formers and this external support around the tube is maintained locally by relative movement of the formers as the bend is developed.
Advantageously, the fixed former has a peripheral groove corresponding to the radius of the required bend around which the tube is formed and the movable former has a peripheral groove co-operable with the groove in the fixed former to provide external support around the section of the tube therebetween. The peripheral groove in the fixed former may be of constant or variable radius and/or may be in one or more planes depending on the desired shape and configuration of the finished bend.
Preferably, the groove in the fixed former is of U-shape in radial cross-section and is a depth sufficient to receive the tube and the marginal edge of the movable former. In this way the movable former is guided for movement around the fixed former to conform the tube to the required configuration. Where the peripheral groove in the fixed former is of variable radius and/or in more than one plane, the movable roller may be adjustable to compensate for such changes so as to follow the profile of the fixed former. For example, the movable former may be spring loaded and/or angularly adjustable.
Advantageously, means is provided for securing the tube in the forming means during the forming operation. For example, the tube may be clamped at a position adjacent to the end of the heated region where the bending operation begins leaving the other end of the tube free for conforming the heated region to the required bend configuration.
Preferably, the means for internally supporting the tube comprises a source of fluid, e.g. compressed air, connectable to the interior of the tube via a valve in one of two seal units for the opposite ends respectively of the tube. Alternatively, the fluid source is connectable to an inflatable bladder positioned within the tube.
Advantageously, the means for cooling the tube comprises a source of fluid, e.g. a gas or liquid, passed over and/or through the tube. Where compressed air is used to provide internal support for the tube during the forming stage, the compressed air may be passed through the tube to cool the tube and maintain the internal support during the cooling stage.